Vertical/Short Take-Off and Landing (V/STOL) aircraft are fixed-wing aircraft that can lift off from the ground and land in a vertical hover, and transition from the vertical hover mode to a horizontal flight mode and back to vertical hover mode. Configurations have included several unsuccessful experimental variants of tailsitter aircraft that launch and land on their tails, vectored-thrust aircraft like the highly successful British AV8B Harrier now built by Boeing Integrated Defense Systems of St. Louis, Mo., a subsidiary of The Boeing Company, Chicago, Ill., and the F-35B Joint Strike Fighter from Lockheed-Martin of Ft. Worth, Tex., that direct the thrust from a jet engine downward to hover and rearward for horizontal flight, and tilt-wing/tilt-rotor aircraft like the V-22 Osprey jointly from Bell Helicopter of Ft. Worth, Tex. and Boeing Integrated Defense Systems of St. Louis, Miss., a subsidiary of The Boeing Company, Chicago, Ill., that directs thrust from two rotatable proprotor assemblies situated in engine nacelles on the ends of the main wing. With tilt-wing aircraft, the entire wing, elevator, and rotor assemblies rotate, resulting in reduced drag in vertical hover mode. With tilt-rotor aircraft, primarily the proprotor/engine nacelle assemblies rotate. In other aspects of flight control and performance, tilt-wing aircraft are essentially the same as tilt-rotor aircraft and, unless the features of tilt-wing aircraft are specifically mentioned, all references to tilt-rotor aircraft herein will apply equally to both tilt-rotor aircraft and tilt-wing aircraft.
Although there have been experimental tilt-rotor aircraft with four proprotor/engine nacelles situated with two proprotor/engine nacelles situated on a small forward wing and two on a small back wing (e.g., the Bell X-22), typical tilt-rotor aircraft like the V-22 Osprey are configured with two proprotor/engine nacelles situated at the end of the main wing. In this configuration, the proprotors are counter-rotating (rotating in opposite directions) to cancel torque between the proprotors. In this configuration, the length of the wing is dictated by the size of the proprotors required to generate the necessary lift. U.S. Pat. No. 5,381,985 to Wechsler et al., Wingtip Mounted Counter-Rotating Proprotor for Tiltwing Aircraft, (Wechsler) discloses a configuration where each proprotor consists of a pair of inline counter-rotating rotors. This permits smaller diameter proprotors that generate the same lift as a single larger rotor, permitting employment of a smaller wing. Additionally, with two rotors in each proprotor, the blade loading on each individual blade is reduced. Tilt-rotor aircraft with only one proprotor are less common. U.S. Pat. No. 6,343,768 to Muldoon, Vertical/Short Take-Off and Landing Aircraft, (Muldoon) discloses a single rotor assembly with a pair of inline counter-rotating rotors situated at the center of the aircraft.
Tilt-rotor aircraft can typically perform in any of three modes: Conventional Take-Off and Landing (CTOL), Short Take-Off and Landing (STOL) or Vertical Take-Off and Landing (VTOL). In Conventional Take-Off and Landing, the proprotors, propellers, or rotors are in a horizontal position, with thrust directed backward, and the aircraft takes of and lands like conventional fixed-wing aircraft, with similar take-off and landing distances. In Short Take-Off and Landing, the nacelles connected to the proprotors, propellers or rotors are angled slightly upward from the horizontal position to direct thrust down and to the rear, leading to shorter take-off and landing distances. In both Conventional and Short Take-Off and Landing the familiar flight controls used by conventional fixed-wing aircraft are employed: horizontal stabilizer for pitch, vertical rudder for yaw, ailerons for roll and wing flaps for take-off and landing. These flight controls rely on the flow of air over the wings (from the horizontal movement of the aircraft) to generate the forces necessary to pitch, roll or yaw the aircraft.
In Vertical Take-Off and Landing the tilt-rotor/tilt-wing aircraft proprotors, propellers, or rotors are in a vertical position, directing thrust downward and giving the tilt-rotor the ability to take-off and land vertically. In this configuration, however, the absence of horizontal motion means that the conventional fixed-wing aircraft control surfaces are useless to control the pitch, roll or yaw of the aircraft. Therefore, in the Vertical Take-Off and Landing mode, the tilt-rotor/tilt-wing aircraft must employ alternative methods of flight control. In one variant, the tilt-rotor/tilt-wing aircraft employs an ability of the nacelles connected to the rotors, propellers, or proprotors to move independently of each other in combination with a variable-pitch proprotor, propeller, or rotor to control flight. The variable-pitch proprotor, propeller, or rotor uses a mechanism to control the pitch of the rotor, propeller, or proprotor blades similar to the variable-pitch propeller mechanism used by conventional fixed wing aircraft. In this case, rotating the nacelles in tandem controls aircraft pitch, such that a slight forward rotation of the nacelles will move the aircraft forward and pitch the aircraft nose down and a slight rearward rotation of the nacelles will move the aircraft rearward and pitch the aircraft nose up. Adjusting the rotor blade pitch of one rotor with respect to the other rotor controls roll by causing one rotor to generate more thrust than the other. Rotating one nacelle forward and the other rearward controls yaw. Using the variable-pitch mechanism means having a moderately complex mechanism at the nacelles connected to the propeller, rotor or proprotor, but a more complex scheme of rotating the nacelles independently.
In another variant, each proprotor or rotor employs a swashplate assembly that permits cyclic and collective blade pitch control similar to a helicopter rotor. Here, forward or rearward cyclic inputs to both proprotors/rotors in tandem controls pitch while applying forward and rearward cyclic input to opposite proprotors/rotors provides yaw control. Adjusting the collective blade pitch of one proprotor/rotor with respect to the other proprotor/rotor controls roll by causing one proprotor/rotor to generate more thrust than the other. This variant simplifies the yaw control by eliminating the need to control the nacelles connected to the proprotor/rotor independently but adds the more complex blade pitch control mechanism, the swashplate. This is the system employed in the Wechsler disclosure.
In single nacelle configurations like the Muldoon disclosure, yaw can not be controlled with collective and cyclic pitch of two propellers, rotors or proprotors. Therefore, Muldoon shows “a series of control surfaces located in the rotor wash” to control yaw (Muldoon, Abstract) in addition to collective and cyclic pitch mechanism on the nacelle assembly for pitch and roll control. In all of the above mentioned configurations, the vertical hover mode requires complex control mechanisms for pitch, roll and yaw control.
Introduction of inline counter-rotating rotors, propellers or proprotors adds complexity because some form of transmission is required to translate the rotation of the engine to the counter-rotation of two rotors, propellers or proprotors. Additionally, two-nacelle configurations like the V-22 Osprey and Wechsler typically employ a transfer case mechanism to provide power to both proprotors, propellers or rotors in the case of an engine failure.
In horizontal flight mode, less power is typically required from the proprotors because the proprotors are freed from the need to generate lift. In horizontal flight mode, lift is generated by the airflow over the wings. For this reason, Muldoon discloses a braking mechanism in the transmission that stops the rotation of one of the proprotors, channeling all the power in to only one proprotor. This improvement in efficiency comes at the cost of an added complexity in the brake mechanism.
Because of all of the complex flight control mechanisms described above, it is not uncommon to implement a fly-by-wire flight control system, as is done on the V-22 Osprey. Such a system must itself be highly complex to smoothly control the conversion from vertical hover mode to horizontal flight mode and the reconversion back to vertical hover mode.
Because of the complexity of the prior art V/STOL aircraft, V/STOL advantages have been realized nearly exclusively by large, complex aircraft. From the foregoing it will be apparent that there is a need for improved flight controls on a tilt-wing/tilt-rotor aircraft when in vertical hover mode that involves less complex mechanisms and is therefore cheaper and more amenable to use on small commercial and private aircraft. Further, it will be apparent that there is a need for a simple mechanism for employing inline counter-rotating propellers, rotors, or proprotors on a tilt-wing/tilt-rotor aircraft. It will also be apparent that there is a need for a simple mechanism for stopping the rotation of one such propeller, rotor or proprotor of an a pair of inline counter-rotating propellers, rotors, or proprotors in horizontal flight mode to improve efficiency. Finally, it will be apparent that a simplified automatic flight control mechanism is needed.